Referring to FIG. 1, a color CRT 10 generally comprises an evacuated glass envelope consisting of a panel 12, a funnel 13 sealed to the panel 12 and a tubular neck 14 connected by the funnel 13, an electron gun 11 centrally mounted within the neck 14 and a shadow mask 16 removably mounted to a sidewall of the panel 12.
A three color phosphor screen 20 is formed on the inner surface of a display window or faceplate 18 of the panel 12. The electron gun 11 generates three electron beams 19a or 19b. Said beams are directed along convergent paths through the shadow mask 16 to the screen 20 by means of several lenses of the gun 11 and a high positive voltage applied through an anode button 15 and being deflected by a deflection yoke 17 so as to scan over the screen 20 through apertures or slits 16a formed in the shadow mask 16.
In the color CRT 10, the phosphor screen 20 formed on the inner surface of faceplate 18, as shown in FIG. 2, comprises an array of three phosphor elements R, G and B of three different emission colors arranged in a cyclic order of a predetermined structure of multiple-stripe or multiple-dot shape and a matrix of light-absorptive material surrounding the phosphor elements R, G and B. And a thin conductive film of aluminum 22 overlies the screen 20 in order to provide a means for applying the uniform potential applied through the anode button 15 to the screen 20, increasing the brightness of the phosphor screen 20, preventing ions from damaging the phosphor screen 20 and preventing from decreasing the potential of the phosphor screen 20. And also, a resin film 22' such as lacquer film may be applied between the aluminum thin film 22 and the phosphor screen 20 to enhance the flatness and reflectivity of the aluminum thin film 22. The resin or lacquer film 22' is to be ignited and volatilized off after that the aluminum thin film 22 have formed onto it.
In a photolithographic wet process, which is well known as a prior art process for forming the phosphor screen 20, a slurry including phosphor particles such as phosphorus components for emission colors or light-absorptive materials is coated on the inner surface of the faceplate 18. It does not, however, meet the higher resolution demands. Moreover, it requires a lot of complicated processing steps and a lot of manufacturing equipments, thereby necessitating a high cost in manufacturing the phosphor screen 20. Also, it discharges a large quantity of effluent such as waste water, phosphor elements, 6th chrome sensitizer, etc., with the use of a large quantity of clean water.
Recently, to solve or alleviate the above problems of said photolithographic wet process, electrophotographic screening process is developed. In wet electrophotographic screening process, however, the above-mentioned problems remains unsolved. What solves or alleviates mostly the above-mentioned problems is the dry electrophotographic screening process.
U.S. Pat. No. 4,921,767, issued to Datta at al. on May 1, 1990, describes one method of electrophotographically manufacturing the phosphor screen assembly using dry-powdered phosphor particles. This invention, however, has some problems such that it requires dark environment during all the steps since the photoconductive layer is sensitive to the visual light, and that its energy consumption is so large because its fixing step comprises infrared radiation for fixing the deposited particles to the photoconductive layer.
The above problem is solved by forming the photoconductive layer with a solution sensitive to the ultraviolet radiation, which is suggested in other inventions assigned to the assignees of the present invention.
For such an example, our Korean patent application Serial No. 95-10420 filed in Apr. 29, 1995 and assigned to the assignees of the present invention describes "Method of manufacturing a screen of a CRT", as is briefly explained in the following.
FIGS. 3A through 3E schematically show various steps in the above-described manufacturing method. FIG. 3A represents a coating step that forms an electrically conductive layer 132 is formed on the inner surface of the faceplate 18 and overlies an photoconductive layer 134 on the conductive layer 132.
The conductive layer 132, for example, can be formed by conventionally applying a volatilizable organic conductive material consisting of about 1 to 50 weight % of a polyelectrolyte commercially known as Catfloc-c, available from Calgon Co., Pittsburgh, Pa., to the inner surface of the faceplate 18 in an aqueous solution containing about 1 to 50 weight % of 10% poly vinyl alcohol and drying the solution. Said conductive layer 132 serving as an electrode for the overlying photoconductive layer 134. The photoconductive layer 134 is formed by conventionally applying to the conductive layer 132, a novel photoconductive solution containing ultraviolet-sensitive material and by drying it.
An example of the ultraviolet-sensitive material can consist of 0.01 to 1 weight % of bis-1,4-dimethyl phenyl(-1,4-diphenyl(butatriene)) or 2 to 5 weight % of tetraphenyl ethylene as a donor, 0.01 to 1 weight % of at least one compound from the group including trinitro-fluorenon (TNF) and ethylanthraquinone (EAQ) as an accepter, 1 to 30 weight % of polystyrene (PS) as a polymeric binder, and balance with solvent such as toluene or xylene. The photoconductive solution is prepared by dissolving 0.01 to 1% by weight of the ultraviolet-sensitive material and 1 to 30% by weight of polystyrene as a polymeric binder in a suitable solvent such as toluene or xylene. The useful compounds as a polymeric binder also may comprise, addition to polystyrene, polyalphamethylstyrene (P.alpha.MS) polymethylmethacrylate (PMMA) and polystyrene-oxalzoline copolymer (PS-OX), et cetera.
FIG. 3B schematically illustrates a charging step, in which the photoconductive layer 134 is charged to a positive potential of less than 1 Volt, preferably above 700 volts by a corona discharger 3b. The charging step does not require a dark environment since the photoconductive layer 134 is sensitive to ultraviolet rays below about 450 nm of wave length.
FIG. 3C schematically shows an exposing step. The shadow mask 16 is inserted in the panel 12 and the positively charged photoconductive layer 134 is selectively exposed through an ultraviolet-transmissive lens system 140 and apertures or slits 16a of the shadow mask 16 to the ultraviolet rays from a ultraviolet lamp 138 with each predetermined incident angle with respect t6 each aperture or slit 16a. The charges of the exposed areas are discharged through the grounded conductive layer 132 and the charges of the unexposed areas remain in the photoconductive layer 134, thus establishing a latent charge image in a predetermined array structure. This exposing step also does not require a dark environment since the ultraviolet rays are used. Three exposures with three different incident angles of the three electron beams, respectively are required for forming a light-absorptive matrix.
FIG. 3D diagrammatically illustrates the outline of a developing step. In conventional developing step of the process such as U.S. Pat. No. 4,921,767, the charging of the dry-powdered particles such as phosphor particles or light-absorptive material is executed by a triboelectrical charging method in which surface-treated carrier beads and phosphor particles, or the carrier beads and light-absorptive material particles are mixed. On the contrary, in the invention according to Korean patent application Serial No. 95-10420 as illustrate in FIG. 3D, the dry-powered particles are suitably charged, and sprayed by compressed air toward the photoconductive layer 134. The dry-powered particles are transferred by compressed air through a venturi tube 146 from a hopper 148 to a nozzle 144b to be sprayed. Below the nozzle 144b, there is provided a discharge electrode 144a such as a corona discharger. The discharge electrode 144a charges the dry-powered particles so that the charged dry-powered particles may be sprayed from the nozzle 144b toward the photoconductive layer 134. The charged dry-powered particles are attracted to one of the areas, exposed or unexposed at said exposing step, on the photoconductive layer 134. The polarity of dry-powered particles charged by the discharge electrode 144a is determined according to on which areas the dry-powered particles are desired to be attached. That is, if the dry-powered particles are desired to be attached to the positive charged, i.e., unexposed areas, they are negatively charged by the discharge electrode 144a. While if the dry-powered particles are desired to be attached to the discharged, i.e., exposed areas, they are positively charged. And hence the dry-powered particles, which are charged positively or negatively and sprayed into the developing container 142, can be attached strong to the surface of the photoconductive layer 134 in a predetermined array pattern due to electrical attraction or repulsion.
FIG. 3E schematically illustrates a fixing step using a liquid electrostatic spray gun. In this fixing step, the surface of the photoconductive layer 134, on which the particles are attached in a predetermined array pattern at said developing step, is sprayed by solvent of petroleum such as xylene, toluene, TCE, methyl isobutyl ketone(MIBK), etc. Then, at least polymers contained in the photoconductive layer 134 are dissolved. And the dry-powdered particles, deposited on the developed areas of the photoconductive layer 134 due to electrical forces, are fixed by adhesion of said dissolved polymers. The vapor swelling method also may be used in this fixing step. In the vapor swelling method, the particles deposited on the developing areas of the photoconductive layer 134 are fixed by being applied to solvent vapor such as acetone, methyl isobutyl ketone.
The steps of charging, exposing, developing and fixing are repeated for the three different phosphor particles of R, G, and B for completing of the manufacturing a CRT. And the black matrix of light-absorptive material particles is formed according to same steps after or before the steps of forming for the three different phosphor particles.
Thus, after the patterns of phosphor particles and the matrix of light-absorptive material have been formed, the resin or lacquer film 22' is formed at a lacquer process conventionally. And then, the aluminum thin film 22 is also formed conventionally at a aluminizing process. After that, the faceplate panel 12 is baked in air at temperature of 425 degrees of centigrade for about 30 minutes. This baking process drives off the volatilizable constituents of solvents, etc., present in the conductive layer 132, the photoconductive layer 134, the respective phosphors R, G, and B, or lacquer film 22', etc. Thereby, the phosphor screen 20 is formed in array pattern of light-absorptive material 21 and three phosphor elements R, G and B as in FIG. 2.
Turning to the developing step of FIG. 3D, when the dry-powdered particles are sprayed from the nozzle 144b, they should have been sufficiently charged with a corona discharge electrode 144a. For the purpose that the particles can be sufficiently charged, the dry-powdered phosphor particles are coated with a first layer of polymethyl methacrylate and a second layer of polyacrylamide. The coating process, however, is very complicated. Also, said two coating layers of polymethyl methacrylate and polyacrylamide of the phosphor particles still do not meet the sufficient chargeability demand. And when not the discharge electrode 144a but a triboelectrical charging method is used as in the developing step described in U.S. Pat. No. 4,921,767, it is additional problem that the carrier beads are required for generating triboelectricity in addition to the coating process for the phosphor particles. Moreover, it is another problem that the developing density of deposited particles on the photoconductive layer 134 is not sufficient by the coated particles with the two layers of polymethyl methacrylate and polyacrylamide.
It is an object of the present invention to provide a method for coating phosphor particles easily and uniformly with high polymers in order to improve chargeability thereof and to improve the developing characteristics onto a photoconductive layer in a dry electrophotographic screening process for a CRT.
It is another object of the present invention to provide phosphor particles coated by the method for coating phosphor particles easily and uniformly with polymers in order to improve chargeability thereof and to improve developing characteristics onto a photoconductive film in a dry electrophotographic screening process for a CRT.
And it is yet another object of the present invention to provide a dry electrophotographic screening process for a CRT using the dry-powered phosphor particles coated by the method for coating phosphor particles easily and uniformly with high-molecular polymers in order to improve chargeability thereof and to improve developing characteristics onto a photoconductive film.